In a word: Jesus.
There are many reasons as to how people choose to live out their faith. For me, I choose to be Catholic because of Christ. I am Catholic because the Jesus that I love and honor and adore chooses to be close to me - especially in the Eucharist.
When Jesus said "This is my body...", offered His life on the cross, and promised to be with us 'till the end of time, He was pointing to the Eucharist - to His Body, Blood, Soul, and Divinity that would remain near and united to us until He returns in glory.
We can meet Jesus anywhere: in others, in creation, and in life's circumstances. We can see Him in the pains and joys of life and we can encounter Him in so many ways. But to actually be one with Him in the Eucharist, as we are one with Him in Heaven, is Heaven on Earth and I am grateful to Jesus for giving us the Church so that she can give us Him. I am grateful for the truths she proclaims in order to uphold the name of Jesus and the teachings He professed. I am grateful for her strong convictions in a world that points us away from Christ. And I am grateful that I can go to her to deepen my understanding of the Lord, for she was instituted by Him.
There are many more reasons why I am Catholic, but I still sum it all up in Jesus. This is why the Church proclaims the Eucharist (Jesus) is the "source and summit of our faith..." because Jesus is and should be the source and summit of our lives.
Thank you, Jesus, for guiding me closer to you by your Holy Church and thank you for giving Yourself to me, through her...